It is known to use closed loop control to furnish DC current to an inductive load, a brushless motor for example, from a DC source by pulse width modulation. A microprocessor determines on the basis of the motor characteristics, feedback of operating parameters and the desired performance, the pulse duty cycle to be applied to a power switching device to attain the desired load current. Where a current control loop measures load current and compares the measured current to a fixed current limit, a loop instability can occur for duty cycles greater then 50%. That occurs since the average load current increases as the duty cycle increases.
The instability is avoided by progressively decreasing the current limit from an initial value at the beginning of each cycle. When the slope of the current limit is properly selected, the average current will remain the same as duty cycle varies, lending stability to the current loop. Prior circuits to accomplish this stabilized control use analog integrated circuitry with external components; these require precision passive components to minimize errors in the system. The exact values of external components depend on manufacturing tolerances and vary with temperature, resulting in a circuit with variable accuracy and controllability.